When single-ended audio amplifiers biased at a fixed reference voltage are turned on, the coupling capacitor of the AC signal at the input of the amplifier is uncharged and at the turn-on instant the charge transient of the input capacitance may cause an undesired typical noise (pop). This disturbance occurs when the final stage of the amplifier is turned on while the charging of the input capacitance is still insufficient.
The phenomenon is highlighted in FIGS. 1A and 1B, in which the functional scheme of a single-ended amplifier and the relative diagrams of the turn-on transient are depicted. If at the instant To the source of the reference voltage Vref is turned on, because of the still incomplete state of charge of the input capacitance Cin, the input voltage Vin (that is, the voltage on the noninverting input (+) of the xA amplifier) will not be equal to the reference voltage Vref applied on the inverting input (-) of the amplifier. If the output power stage of the amplifier is turned on, for example, at the instant T1 (which may coincide with the turn-on instant To), the signal output is equal to Vout=A.times.Vref exp(-t/(Rin*Cin)) and causes a typical popping noise.
The level of this popping noise is commonly reduced by delaying the turn-on of the final stage of the amplifier in respect to the turn-on instant of the source of the reference voltage Vref. However, this approach cannot completely overcome the problem unless the delay is very long. Another known technique includes doubling the input stages, in order to "mute" the amplifier during the turn-on transient, however more complex circuits are needed and an appropriate time constant is programmable through a dedicated pin of the device to retard the switching from a mute to a play condition.